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Abstract:

A method for growing a silicon carbide single crystal on a single crystal
substrate comprising the steps of heating silicon in a graphite crucible
to form a melt, bringing a silicon carbide single crystal substrate into
contact with the melt, and depositing and growing a silicon carbide
single crystal from the melt, wherein the melt comprises 30 to 70 percent
by atom, based on the total atoms of the melt, of chromium and 1 to 25
percent by atom, based on the total atoms of the melt, of X, where X is
at least one selected from the group consisting of nickel and cobalt, and
carbon. It is possible to improve morphology of a surface of the crystal
growth layer obtained by a solution method.

Claims:

1. A method for growing a silicon carbide single crystal on a single
crystal substrate comprising the steps of heating silicon in a graphite
crucible to form a melt, bringing a silicon carbide single crystal
substrate into contact with the melt, and depositing and growing a
silicon carbide single crystal from the melt, wherein the melt comprises
30 to 70 percent by atom, based on the total atoms of the melt, of
chromium and 1 to 25 percent by atom, based on the total atoms of the
melt, of X, where X is at least one selected from the group consisting of
nickel and cobalt, and carbon.

2. The method according to claim 1, wherein the melt comprises 3 to 7
percent by atom, based on the total atoms of the melt, of X.

3. The method according to claim 1, wherein the melt is prepared by adding
Si, Cr and X as raw materials in a graphite crucible, melting the raw
materials to form an alloy, and heating the alloy to a temperature higher
than a solidus temperature of the alloy.

4. The method according to claim 1, wherein at least part of carbon in the
melt originates from the graphite crucible.

5. The method according to claim 1, wherein all of carbon originates from
the graphite crucible.

6. The method according to claim 1, wherein the single crystal substrate
has the same crystal form as the silicon carbide single crystal to be
grown.

7. The method according to claim 1, wherein used for growing a bulk single
crystal.

Description:

FIELD OF INVENTION

[0001]The present invention relates to a new method for growing a silicon
carbide single crystal by a solution method, more particularly to a
method for growing a silicon carbide single crystal by a solution method
using a new melt (sometimes referred to as a solution), that is subjected
to improve morphology of a crystal growth surface and increase a crystal
growth rate.

BACKGROUND OF INVENTION

[0002]A silicon carbide (SiC) single crystal has superior properties such
as thermal and chemical stability, excellent mechanical strength, good
resistance to radiation, and dielectric breakdown voltage and thermal
conductivity higher than Si. It is also characterized in that it is easy
to electronically control p and n conductivity types by doping an
impurity, as well as it has a wide band gap (about 3.0 eV for single
crystal 6H--SiC, about 3.3 eV for single crystal 4H--SiC). Therefore, it
can achieve high temperature, high frequency, resistance to voltage and
resistance to environment, which cannot be achieved by any existing
semiconductor material, such as silicon (Si) and gallium arsenide (GaAs).
It is increasingly expected as a next-generation semiconductor material.

[0003]Heretofore, a vapor phase method, Acheson method, and a solution
method are known as typical methods for growing a silicon carbide single
crystal.

[0004]Typical examples of vapor phase methods include a sublimation method
and a chemical vapor deposition (CVD) method. In a sublimation method,
various defects tend to be formed in a crystal and a crystal tends to be
poly-crystallized. In a CVD method, because raw materials are limited to
gases, a formed crystal is a thin film and therefore it is difficult to
produce a bulk single crystal.

[0005]Furthermore, in Acheson method, since silica stone and coke are used
as raw materials and heated in an electric furnace, it is impossible to
obtain a high-purity product because of impurities contained in the raw
materials.

[0006]And, the solution method is a method that comprises melting
silicon-containing alloy in a graphite crucible, dissolving carbon from
the graphite crucible into the melt, growing silicon carbide crystal
layer on a seed crystal substrate placed at the cold area by solution
deposition. And, it is known that the solution method has low growth rate
but it is advantageous as a method to obtain a bulk crystal.

[0007]For this reason, recently, various studies have been done to enhance
a growth rate for growing a silicon carbide single crystal by a solution
method that has not above-mentioned problems in a vapor phase method and
Acheson method.

[0008]Japanese Unexamined Patent Publication No. 2000-264790 describes a
method for producing a silicon carbide single crystal, wherein the method
comprises melting a raw material containing at least one element of
transition metals, Si, and carbon to form a melt, bringing the melt into
contact with a silicon carbide seed crystal as well as cooling the melt
to the temperature lower than the liquidus-line temperature of the melt,
depositing and growing the silicon carbide single crystal. And, although
it discloses Fe, Co, Ni (group VIII), Ti, Zr, Hf (group IVb), V, Nb, Ta
(group Vb), Cr, Mo and W (group VIb) as transition metals, it discloses
only a composition comprising Mo, Cr, and Co. However, with respect to
the quality of a depositing single crystal, a measuring method or
identifying means is not disclosed, and a macro defect of the crystal
growth surface is not recognized.

[0009]Japanese Unexamined Patent Publication No. 2004-2173 describes a
method for producing a silicon carbide single crystal, comprising the
steps of immersing a seed crystal substrate of silicon carbide into an
alloy melt that contains Si, C and M, where M is Mn or Ti, has M/(Si+M)
atomic ratio (X) of 0.1 to 0.7 when M is Mn and 0.1 to 0.25 when M is Ti,
and does not contain unmelted C, and growing a silicon carbide single
crystal on the seed crystal substrate by putting silicon carbide into
super-saturation state by super-cooling the alloy melt in the periphery
of the seed crystal substrate. In addition, it described that silicon
carbide tends to be poly-crystallized by carbon that was charged as a raw
material, concerning the method for producing a silicon carbide single
crystal described in Japanese Unexamined Patent Publication No.
2000-264790.

[0010]Japanese Unexamined Patent Publication No. 2006-143555 describes a
method for producing a silicon carbide single crystal, wherein the method
comprises immersing a seed crystal substrate of silicon carbide into a
alloy melt that contains Si, C and M, where M is Fe or Co, and has the
value of [M]/([M]+[Si]), wherein [M] expresses molar concentration of M;
[Si] expresses molar concentration of Si, not less than 0.2 and not more
than 0.7 when M is Fe, or not less than 0.05 and not more than 0.25 when
M is Co, growing a silicon carbide single crystal on the seed crystal
substrate by putting the alloy melt in the periphery of the seed crystal
substrate into super-saturation state of silicon carbide. However, a
macro defect of the crystal growth surface is not recognized.

[0011]Japanese Unexamined Patent Publication No. 2007-76986 describes a
method for producing a silicon carbide single crystal, wherein the method
comprises bringing a single crystal substrate for growing silicon carbide
contact with a melt that contains Si, Ti, M and C, where M is Co and/or
Mn and has the following atomic ratio of Si, Ti and M:
0.17≦[Ti]/[Si]≦0.33 and
0.90≦([Ti]+[M])/[Si]≦1.80, where [Ti] expresses molar
concentration of Ti, [M] expresses molar concentration of M, or that
contains Si, Ti, M and C, where M is Al, and has the following atomic
ratio of Si, Ti and M, wherein, 0.17≦[Ti]/[Si]≦0.33 and
0.33≦([Ti]+[M])/[Si]≦0.6, where [Ti] expresses molar
concentration of Ti, [Si] expresses molar concentration of Si, [M]
expresses molar concentration of M, growing a silicon carbide single
crystal on the single crystal substrate by putting the silicon carbide
dissolved in the melt into super-saturation state by super-cooling the
melt in the periphery of the single crystal substrate. However, the macro
defect of a grown crystal surface is not recognized.

SUMMARY OF INVENTION

[0012]As mentioned above, in the methods for growing a silicon carbide
single crystal by the solution method described in the heretofore known
literature, the macro defect of a grown crystal surface was not
recognized, and it was impossible to improve morphology of a surface of
the crystal growth layer.

[0013]As a result of study about the method for growing a silicon carbide
single crystal by a solution method, we have found that a relatively
large growth rate is obtained by using a Si--Cr--C melt comprising not
less than certain amount of Cr, but, that the surface of the silicon
carbide single crystal growth layer obtained is unstable when Si--Cr--C
melt is used, that small variation of growing conditions negatively
impact the surface of the growth layer, in other words, that morphology
(configuration) of a surface of grown crystal is not sufficient, and
therefore it may affect the quality of grown crystal obtained.

[0014]The object of the present invention is to provide a method for
growing a silicon carbide single crystal by a solution method to improve
morphology of a surface of a crystal growth layer.

[0015]The present invention relates to a method for growing a silicon
carbide single crystal on a single crystal substrate comprising the steps
of heating silicon in a graphite crucible to form a melt, bringing a
silicon carbide single crystal substrate into contact with the melt, and
depositing and growing a silicon carbide single crystal from the melt,
wherein the melt comprises 30 to 70 percent by atom, based on the total
atoms of the melt, of chromium and 1 to 25 percent by atom, based on the
total atoms of the melt, of X, where X is at least one selected from the
group consisting of nickel and cobalt, and carbon.

[0016]According to the present invention, it is possible to achieve the
improvement of morphology of the surface of a crystal growth layer and to
grow a silicon carbide single crystal at a growth rate that is the same
as or higher than the solution method described in the heretofore known
literature.

BRIEF DESCRIPTION OF DRAWINGS

[0017]FIG. 1 depicts an embodiment of a production equipment to carry out
the method of the present invention.

[0018]FIG. 2 depicts an equipment by which growth experiment for a silicon
carbide single crystal was carried out in each example.

[0019]FIG. 3A shows a picture of morphology of a surface of the crystal
growth layer of silicon carbide crystal obtained in comparative example
1, wherein the composition atomic ratio of Si:Cr is 50:50.

[0020]FIG. 3B shows a picture of morphology of a surface of the crystal
growth layer of silicon carbide crystal obtained in comparative example
1, wherein the composition atomic ratio of Si:Cr is 60:40.

[0021]FIG. 4 shows a picture of morphology of a surface of crystal growth
layer of silicon carbide crystal obtained in example 1.

DESCRIPTION OF EMBODIMENTS

[0022]The present invention is illustrated referring to FIG. 1 that
depicts an embodiment of a production equipment to carry out the method
of the present invention.

[0023]In FIG. 1, the growth of silicon carbide single crystal is carried
out using graphite crucible 5 surrounded with heat insulator 6 as a
reaction vessel. The growth of a silicon carbide single crystal can be
achieved by bonding and fixing single crystal substrate 4 comprising
silicon carbide single crystal on the tip of graphite bar 3 (also
referred to as graphite axis) that is an example of silicon carbide seed
crystal supporting member, and dipping this into melt 2, wherein melt 2
is heated by high frequency coil 1 as a heating equipment, growing a
single crystal substrate 4.

[0024]In the present invention, it is necessary in the method for growing
a silicon carbide single crystal on a single crystal substrate comprising
the steps of heating silicon in a graphite crucible to form a melt,
bringing a silicon carbide single crystal substrate into contact with the
melt, and depositing and growing a silicon carbide single crystal from
the melt, that the melt comprises 30 to 70 percent by atom, based on the
total atoms of the melt, of chromium and 1 to 25 percent by atom, based
on the total atoms of the melt, of X, where X is at least one selected
from the group consisting of nickel and cobalt, and carbon.

[0025]Without use of both Cr and X, for example, three elements Mo--Si--C,
three elements Cr--Si--C, three elements Co--Si--C that are described as
concrete examples in Japanese Unexamined Patent Publication No.
2000-264790, are expected to improve growth rate, but may provide poor
quality of depositing crystal.

[0026]Furthermore, in the Si--Cr--X--C melt, growth rate of the silicon
carbide single crystal will be particularly low when Cr is less than 30
percent by atom, polycrystal forms around the silicon carbide single
crystal and it comes to be very difficult to grow only a single crystal
stably when Cr is more than 70 percent by atom. Therefore, these cases
are not proper. Furthermore, in the Si--Cr--X--C melt, morphology of
surface of a silicon carbide single crystal is not improved when X is
lower than 1 percent by atom, part or all of the silicon carbide crystal
obtained poly-crystallizes when X is more than 25 percent by atom, and
then it turns difficult to grow a single crystal stably, so these cases
are not preferable.

[0027]In the present invention, the reason why the growth rate of silicon
carbide single crystal is increased and morphology of crystal surface is
improved by using the Si--Cr--X--C melt of said composition is believed
that Cr improves dissolution of C (carbon) from the graphite (crucible in
FIG. 1) with which the melt liquid contacts, this C will become a raw
material of silicon carbide crystal as a result, and X will reduce energy
of solid-liquid interface or surface energy of melt (solution).

[0028]There are no restrictions on a method for preparing the Si--Cr--X--C
melt having the aforementioned composition and for obtaining a silicon
carbide single crystal in the method of the present invention. For
example, first Si, Cr and X are added as raw materials in a graphite
crucible in a reaction vessel, the raw materials are melted and heated to
the temperature higher than the solidus temperature of the produced alloy
to form a melt. In addition, at least part of C in the Si--Cr--X--C melt
originates from the graphite crucible, and it is especially preferable
that all of C originates from the graphite crucible. Furthermore, part of
C may be charged as a raw material that is carbide or carbon. And part of
C may be supplied to the melt by blowing a gas containing carbon, such as
methane, into the melt.

[0029]Heating of the melt is continued. Raw materials consisting of Si, Cr
and X are melted sufficiently and C is sufficiently dissolved. Carbon
concentration in the melt generated comes to near the saturating
concentration of silicon carbide in the melt as a solvent. When the
carbon concentration becomes constant, a seed crystal substrate for
silicon carbide growth is brought to contact with the melt. A silicon
carbide single crystal grows on the single crystal substrate by putting
silicon carbide melted in the melt into super-saturation state by
super-cooling the melt in the periphery of the seed crystal substrate to
the temperature not higher than 2100° C., especially approximately
1600 to 1800° C. with the use of temperature gradient method in
which the melt has temperature gradient of, for example, approximately 5
to 50° C./cm or with the use of cooling process that cools the
melt by operating the heating equipment.

[0030]It is preferable to use a single crystal seed having the same
crystalline form as the crystal to be grown. For example, a single
crystal of silicon carbide made by sublimation method can be used.

[0031]In the method of the present invention, manufacturing procedure that
is heretofore known in itself in the solution method, for example,
graphite crucible shape, heat method, heating time, atmosphere, rate of
temperature increase and rate of cooling, can be applied.

[0032]For example, heat method may include high-frequency induction
heating. Heating time (approximate time from charging of the raw material
before SiC saturating concentration is reached) may include approximately
from a several hours to 10 hours depending on the size of a crucible (for
example, approximately from 3 to 7 hours). Atmosphere may include rare
gas, inert gas such as He, Ne, Ar, or combination of the above-mentioned
gas and N2 or methane.

[0033]According to the method of the present invention, it is possible to
produce a silicon carbide single crystal, preferably n-type silicon
carbide single crystal that does not substantially include polycrystal,
with the growth rate that is the same as or higher than the previously
known growth method of a silicon carbide single crystal by the solution
method of three-components system (for example, Si--Cr--C melt system) or
four-components system (for example, Si--Ti--Al--C melt system,
Si--Ti--Mn--C melt system, Si--Ti--Co--C melt system).

[0034]In addition, according to the method of the present invention, it is
possible to produce a silicon carbide single crystal that can achieve the
improvement of morphology of a surface of crystal growth layer.

[0035]The method of the present invention is of course applicable to the
growth method of a bulk single crystal. It is also applicable to the
surface of silicon carbide substrate in liquid phase epitaxial growth
layer forming technique.

EXAMPLES

[0036]The following examples illustrate the present invention.

[0037]In the following each example, growth experiment of a silicon
carbide single crystal was carried out using the equipment in which a
graphite crucible depicted in FIG. 2 was used as a reaction vessel.
Further, a graphite bar 3 had a W--Re thermocouple in it, and a radiation
thermometer 8 was placed at a graphite crucible 5.

[0038]To the graphite crucible 5, Si was added, and then Cr and X were
added together at the same time. The preset temperature from 1800 to
2100° C. was maintained for 2 to 3 hours by heating continuously.
C was dissolved into the melt 2 from graphite crucible 5 to reach the
silicon carbide saturating concentration. A silicon carbide single
crystal substrate 4 that was fixed on the tip of graphite bar 3 was
dipped into the melt 2. After maintaining the preset temperature, a
silicon carbide single crystal grew on the single crystal substrate 4 by
setting a temperature gradient to the melt of 0.8 to 3.0° C./mm
between the single crystal substrate 4 and the front surface of growing
crystal (not shown), by operating a high frequency coil 1 that is a
heating equipment. After a lapse of growth time, grown crystal was pulled
out completely from the melt 2 and the graphite crucible 5 was cooled
slowly to room temperature, then grown silicon carbide single crystal was
obtained.

[0039]Concerning silicon carbide crystal obtained in each example,
morphology of a surface of the crystal growth layer was observed by eyes
and by microscope. In addition, whether a silicon carbide crystal
obtained in each example was a single crystal or polycrystal was
confirmed by X-ray (XRD).

Comparative Example 1

[0040]A raw material comprising 45 percent by atom of Si and 45 percent by
atom of Cr was added in the graphite crucible, heated and melted. After
maintaining a constant temperature, crystal growth was carried out by
dipping a single crystal substrate into the melt. The silicon carbide
crystal obtained was confirmed to be a single crystal.

[0041]Measurement of temperature of the melt etc. was carried out using a
radiation thermometer and a thermocouple. The radiation thermometer was
installed at the observation window above the liquid surface, wherein
from the window the liquid surface could be observed directly, it was
possible to determine the temperature before and after contact with melt.
In addition, the temperature just after contact with the melt was
determined by using the thermocouple installed inside the graphite bar,
on which a single crystal substrate was attached, at the position of 2 cm
apart from the single crystal substrate.

[0042]Growth rate of silicon carbide single crystal was 210 μm/h.

[0043]Furthermore, pictures of morphology of surfaces of crystal growth
layer are shown in FIG. 3A and FIG. 3B. FIG. 3A shows a picture in the
case that the atomic ratio of Si:Cr is 50:50, FIG. 3B shows a picture in
the case that atomic ratio of Si:Cr is 60:40. From FIG. 3A and FIG. 3B,
when using a Si--Cr--C melt, morphology of many steps appeared on the
growth surface of the silicon carbide single crystal, and it is found
that morphology of the surface is poor.

Example 1

[0044]A raw material comprising 50 percent by atom of Si and 45 percent by
atom of Cr and 5 percent by atom of Ni was added in graphite crucible 5,
heated and melted. After maintaining a constant temperature, a crystal
growth was carried out by dipping a single crystal substrate into the
melt. The silicon carbide crystal obtained was confirmed to be a single
crystal.

[0045]Measurement of temperature of the solution etc., observation of
morphology, measurement of growth rate of silicon carbide single crystal
were carried out in the same manner as in Comparative Example 1.

[0046]Growth rate of SiC single crystal was 240 μm/h.

[0047]Furthermore, a picture of morphology of the surface of a crystal
growth layer is shown in FIG. 4. From FIG. 4, when using a Si--Cr--Ni--C
melt, morphology of the growth surface of silicon carbide single crystal
was found to be significantly improved.

Comparative Example 2

[0048]Crystal growth was carried out in the same manner as in Example 1
except that the raw material consisting of Si, Ti and Al, ratio of Al
being altered in the range between 0 and 10 percent by atom, was added in
the graphite crucible 5, heated and melted, maintained at a constant
temperature (about 1810° C.), a single crystal substrate was
dipped into the melt.

[0049]In the Si--Ti--Al--C melt system, growth rate of crystal was not
more than 140 μm/h even though Al ratio of the total composition was
altered.

Example 2

[0050]A raw material comprising 50 percent by atom of Si and 45 percent by
atom of Cr and 5 percent by atom of Co was added in the graphite crucible
5, heated and melted. After maintaining a constant temperature, crystal
growth was carried out by dipping a single crystal substrate into the
melt. The silicon carbide crystal obtained was found to be a single
crystal.

[0051]Measurement of temperature of the solution etc., observation of
morphology, measurement of growth rate of silicon carbide single crystal
were carried out in the same manner as in Comparative Example 1.

[0052]Growth rate of silicon carbide single crystal was 225 μm/h.

[0053]Furthermore, a picture of morphology of growth surface of a crystal
growth layer was same as FIG. 4. From this result, when using a
Si--Cr--Co--C melt, morphology of the growth surface of silicon carbide
single crystal was found to be significantly improved.

Comparative Example 3

[0054]Crystal growth was carried out in the same manner as in Example 1
except that the raw material consists of Si and Cr without adding Ni,
ratio of Cr being altered in the range between 3 and 95 percent by atom,
was added in the graphite crucible 5, heated and melted, maintained at a
constant temperature (about 1980° C.), a single crystal substrate
was dipped into the melt.

[0055]Morphology of the surface of a silicon carbide crystal growth layer
was poor as same as Comparative Example 1, and part or all of silicon
carbide obtained was found to be poly-crystallized when ratio of Cr in
the total amount of Si and Cr was more than 70 percent by atom.

INDUSTRIAL APPLICABILITY

[0056]The method for growing silicon carbide single crystal of the present
invention will make it possible to obtain a single crystal of silicon
carbide that may have possibility to achieve high temperature, high
frequency, resistance to voltage and resistance to environment, therefore
has possibility as a next generation semiconductor material.

[0057]In addition, the method for growing silicon carbide single crystal
of the present invention will make it possible to achieve the improvement
of morphology of a surface of silicon carbide grown crystal.

[0058]Furthermore, the method for growing silicon carbide single crystal
of the present invention will make it possible to grow a silicon carbide
single crystal with the growth rate that is same as or higher than the
heretofore known solution method.